Figure 1. Exposure of Unit Rg, showing meter-thick black glass Figure 2. Cliff face exposure of the 1-kilometer-long glass body. layers interleaved with crystalline rhyolite. Lighter colored rock The glass body is outlined in black ink for emphasis. To its left are bordering the black glass is composed of devitrified glass. steeply dipping layers of Unit Lag. The scale bar is approximately 10 meters. tures accompany the folding, suggesting that the rocks were example. We believe that the Butcher Ridge Igneous Complex warm when deformed. The nature of the folding suggests that is unique. It may represent mixing in a hypabyssal setting of this deformation developed during the emplacement of these two magmas, one silicic, derived by melting of crustal rocks, rocks and is not tectonic in origin. and one basaltic (precursor of Ferrar Dolerite), derived from As noted earlier, black glass occurs as layers interleaved the mantle. with rhyolite and andesite. In addition, glass forms a 1-kilo- This work was supported by National Science Foundation meter-long body which crops out on the east side of the ridge. grant DPP 77-21590. The layers of adjacent Unit La have been folded into a broken, hinged anticline which is draped around the glass body. Along References the margins of this body, layers of andesite are engulfed by the glass (figure 2). Grindley, C. W., and Laird, M. C. 1969. Geology of the Shackleton The glassy textures of the rocks necessitated the use of vol- Coast (Folio 12, Plate 14). In V. C. Bushnell and C. Craddock (Eds.), canic terminology; however, there is no indication that the Geologic maps of Antarctica, Antarctic map folio series. New York: individual layers represent individual flows. Among other American Geographical Society. Kyle, P. R., Elliot, D. H., and Sutter, J evidence, we found no brecciated surfaces or weathering . F. 1981. Jurassic Ferrar Super- group tholeiltes from the Transantarctic Mountains, Antarctica, and horizons between individual layers. We are not aware of tex- their relationship to the initial fragmentation of Gondwana. In tural layering on the scale elsewhere; furthermore, the kilo- M. M. Cresswell and P. Vella (Eds.), Gondwana five. Rotterdam: meter-long glass body is possibly larger than any other known A. A. Balkema.

Geological investigations in the During the 1980-81 field season our party undertook detailed geological mapping from two base camps in the La La Gorce Mountains and central Gorce Mountains and traversed Scott Glacier (85°45S Scott Glacier area 153°00W) for reconnaissance sampling of the granitic rocks found throughout much of the area. The party was landed by an LC-130 aircraft on Robison Glacier, 14 December 1980 (fig- EDMUND STUMP, STEPHEN SELF, JERRY H. SMIT, ure 1). We established our first camp to the east of Ackerman PHILIP V. COLBERT, and TERRY M. STUMP Ridge and then moved to the second site on 1 January 1981; mapping of the La Gorce Mountains was completed from these Department of Geology two camps. We returned to the landing site on 7 January to Arizona State University resupply and on 10 January sledded down the east side of Scott Tempe, Arizona 85281 Glacier to the Gothic Mountains, where we established a third

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firmed the comformable relationship, but suggested that the vem SZABO BLUFF volcanics overlie the La Gorce Formation. After careful study of the contact this past season, we have concluded that the two 3 formations are bounded by a fault. Within several hundred meters of this fault the rocks are highly sheared, with the shearing in part parallel to bedding in the La Gorce Formation, thus producing the apparently conformable contact. Kink folds N is / with variable orientations occur throughout a wide zone adja- n $) - JFAULT \ \M - cent to the fault. Tops are indicated toward the south in both (tfl)b- MOON the La Gorce Formation and the volcanic formation, but the fault boundary precludes a determination of which rocks are //A:?41"( \\\SS\\ older. \\ The section of the volcanic formation adjacent to the fault 7/\4/19 contains volcanic flows interbedded with sedimentary units, including massive sandstones, some containing sparse con- glomerate, purple and green shales, and volcaniclastic rocks. \ Farther north, to the end of Ackerman Ridge, the rocks are a I NTRUSI massive, silicic porphyry similar to the Wyatt Formation found on the west side of Scott Glacier. The sedimentary rocks in the Ackerman Ridge section are unique among outcrops of the late Precambrian volcanic rocks in the region and are also distinctly different from the graywacke-shale association of the La Gorce Formation. We found that most of the western portion of the La Gorce Mountains is underlain by silicic porphyry similar to that on

0 • I •• 20km 1^ 0 lOuni 1ç 148• 146 ( 144

S Figure 1. Sketch map of La Gorce Mountains area. LS = landing site; 1 = first base camp; 2 = second base camp.

base camp (figures 2 and 3). The 72-kilometer traverse down the glacier took 13 hours, compared to 9 hours on the return to the landing site on 16 January. The season ended 22 January when we were airlifted back to McMurdo. Minshew (1967), the first geologist to work in the La Gorce Mountains, identified the late Precambrian La Gorce Forma- tion, a sequence of graywacke and shale deposited as turbi- \cfr -- dites. Our party found numerous criteria to support Mm- Fig.1 shews interpretation of depositional mode. Observed sedi- 4 - mentary structures include graded beds, flute and load casts, convolute bedding and flame structure, and low-angle cross- I bedding. Much of the bedding is massive, and fine lamina- cf . tions are also common. Incomplete Bouma sequences occur throughout the section. The La Gorce Formation is deformed into tight chevron and % 4& isoclinal folds with steep axial planes and variably plunging V/ / axes. The folding occurs in zones distributed between portions of relatively undeformed beds which strike approximately east-west and dip steeply to the south. Volcanic rocks similar to the late Precambrian Wyatt For- I/I mation (Borg 1980; Minshew 1967; Murtaugh 1969), which / occurs on the west side of upper Scott Glacier and in the Reedy o 30km Glacier area, were found by Katz and Waterhouse (1970) to be 0 1550 in conformable contact with the La Gorce Formation on Ack- 20mi 14;° erman Ridge. These investigators also suggested that the La Gorce Formation is stratigraphically younger than the volcan- ics. Stump (1976) visited the contact briefly in 1971 and con- Figure 2. Sketch map of Scott Glacier area. 3 = third base camp.

56 ANmRcTIc JOURNAL Granitic rocks were systematically collected on the traverse down Scott Glacier for later chemical analysis. In the Gothic Mountains a roof pendant of porphyry similar to the Wyatt Formation was visited and the granitic rocks of the area were collected. The northeastern portion of the Gothic Mountains is underlain by a striking granite prophyry with potassium- feldspar crystals up to 15 centimeters throughout the pluton. On the last day in the field we discovered a pegmatite con- taining yellow concentrations of calcium/uranium-bearing hydrated silicates at Szabo Bluff northeast of the landing site. Funding for this project was provided by National Science Foundation grant DPP 78-20624. During this season we had excellent support from all aspects of the U.S. Antarctic Research Program. We are particularly grateful to LCDR Jack Paulus for our pickup landing in whiteout conditions.

Ajwt References

Borg, S. G. 1980. Petrology and geochemistry of the Wyatt Formation and the Queen Maud batholith, upper Scott Glacier area, Antarctica. Unpublished masters thesis, Arizona State University. Figure 3. Camp 3 with granitic rocks of the Organ Pipe Peaks, Katz, H. R., and Waterhouse, B. C. 1970. Geological reconnaissance Gothic Mountains, In the background. of the Scott Glacier area, south-eastern Queen Maud Range, Ant- arctica. New Zealand Journal of Geology and Geophysics, 13, the end of Ackerman Ridge. The contact between this rock and 1030-1037. the La Gorce Formation is at the head of the central valley Minshew, V. H. 1967. Geology of the Scott Glacier and Wisconsin Range across 10 kilometers of ice-cored moraine, where the porphyry areas, central Transantarctic Mountains, Antarctica. Unpublished intrudes the sedimentary rocks, making it clearly younger. If doctoral dissertation, Ohio State University. Murtaugh, J G. Geology of the Wisconsin Range batholith, the porphyry in the western La Gorce Mountains is of the same . 1969. Transantarctic Mountains. New Zealand Journal of Geology and Geo- magmatic episode that produced the volcanics on Ackerman physics, 12, 526-550. Ridge, then those rocks also must be younger than the La Stump, E. 1976. On the late Precambrian-early Paleozoic metavolcanic Gorce Formation. Mount Mooney is composed of similar silicic and metasedimentary rocks of the Queen Maud Mountains, Antarctica, porphyry and contains a pluton of an unusual, tourmaline- and a comparison with rocks of similar age from southern Africa (Report bearing granite at its northern end. 62). Columbus: Ohio State University, Institute of Polar Studies.

18.6-year modulation tide at the longitude, due to the asymmetric distribution of the Earths mass. The bulge at temperate latitudes does not move relative South Pole to the Earth with exact 12-hour and 24-hour periods because of the non -station arity of the Moons orbit. The line of nodes of the Moons orbit rotates with an 18.6-year period; thus, the P. A. RYDELEK, L. KNOPOFF, and W. ZUirj amplitudes of the daily and semidaily tides are modulated with an 18.6-year period. Institute of Geophysics and Planetary Physics We have observed a significant variation in amplitude of University of California the diurnal and semidiurnal tidal components 01, K1 , and M2 Los Angeles, California 90024 at the South Pole which, we have indicated, is due to the nonuniform distribution of matter of the Earth about the axis of rotation. The six years (1970-71, 1974, 1977-79) of obser- Diurnal and semidiurnal gravity tides should be absent at vations with stable LaCoste-Romberg tidal gravimeters at the South Pole if the mass in the solid Earth and its oceans is Amundsen-Scott Station were uninterrupted by significant distributed symmetrically about the axis of rotation. We have instrumental or operational failures. Harmonic analyses of observed that these tidal components are not zero at the South selected long runs of hourly gravity values from these 6 years Pole (Jackson and Slichter 1974; Knopoff and Rydelek 1980). of recordings have shown that the diurnal tidal components These terms arise because the tidal bulge, which is concen- 01 and K1 decreased from 1970 to 1979 while the semidiurnal trated between the tropics, has an amplitude that varies with component M2 increased over this same period. The variation

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